Gas turbine fuel system with regenerator temperature compensation



March 1961 E. E. FLANIGAN EILALI 2,976,683

GAS TURBINE FUEL SYSTEM WITH REGENERATOR TEMPERATURE COMPENSATION FiledSept. 8, 1958 3 Sheets-Sheet 1 .W 36 7' if a ATTORNQ 28 1961 F N] N AL2,976,683

GAS TUR E FUE YS WI REGENERATOR TEMPERATURE COMPENSATION Filed Sept. 8,1958 3 Sheets-Sheet 2 March 28, 1961 GAS TUR Filed Sept. 8, 1958 E. E.FLANIGAN ETAI. 2,976,683 BINE FUEL SYSTEM WITH ENERATOR TEMPERATURECOMPENSAT 5 Sheets-Sheet 3 INVENTOR.

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GAS TURBINE FUEL SYSTEM WITH REGENER- ATOR TEMPERATURE COMPENSATIONEugene E. Flanigan, Detroit, and Richard M. Zeek, Utica, Micl1.,assignors to General Motors Corporation, Detr'oit, Mich., a corporationof Delaware Filed Sept. 8, 1958, Ser. No. 759,689

6 Claims. (Cl. 60-3928) Our invention relates to fuel supply and controlsysterns for gas turbine engines, particularly for engines including aregenerator which transfers heat from the turbine exhaust gases to thecombustion air leaving the compressor of the engine. A regenerative gasturbine presents certain problems in the supply and control of fuelwhich distinguishes it from the usual non-regenerative turbine. Theseproblems are intensified when the The present invention was conceivedfor use with a regenerative gas turbine of approximately 250 shafthorsepower.

Because of the high heat storage capacity of regenerative heatexchangers, the control of fuel, particularly in starting the engine,needs to take into account, at least in a general way, the temperatureof the regenerator. More specifically, if the engine is started after ithas been out of use for a considerable time, the regenerator will becold. On the other hand, if the engine is started when it has been shutdown for only a few seconds or minutes, the regenerator may be quitehot. In this case, much less fuel is needed to operate the engine thanwhen the regenerator is cold. One feature of the invention is a thermalcompensating means sensitive to the temperature or heat output of theregenerator which acts to provide additional fuel, above that normallyscheduled, when the regenerator is cold.

Other features of the invention lie in improvements in the means forinjecting or atomizing fuel. The fuel flow under idling conditions ofsuch an engine is very small, making proper atomization of fueldifficult to achieve. Air-atomizing nozzles are preferable under suchconditions. While nozzles of this type are well known, the inventionincludes features which improve the perforrnance of the nozzles andsimplify the means for supplying atomizing air to them. We have foundthat air at a sufficient pressure differential above that in thecombustion chambers can be taken from the compressor of the engine by asuitably disposed probe. By using such means to provide the atomizedair, the usual auxiliary compressor or pressure source may be dispensedwith in normal operation.

Another difficulty experienced with the fuel nozzles arises from thefact that combustion air in a regenerative turbine is quite hot becauseof heating in the regenerator. Since the fuel flow is small, it does notsuffice to cool the nozzle adequately, and trouble has been experiencedwith fuel vaporizing in the nozzles. Such vaporizing of fuel reduces theflow through the fuel nozzle. Reduced flow to any one nozzle resultingfrom vaporizing still further reduces the cooling of the nozzle by thefuel, so that the unbalance of fuel flow tends to worsen progressively.The fuel system of this invention cures this fault of previous systemsby providing a cooler for each fuel nozzle which takes heat from thenozzle by direct heat exchange to prevent the temperature of the nozzlefrom rising to the boiling point of the fuel. Preferably, the fluid forcooling'the nozzle is fuel supplied by the engine fuel pump 2,976,683Fatented Mar. 28, 196i ice in excess of that required for operation ofthe engine.

The nature and advantage-s of the invention will be apparent to thoseskilled in the art from the succeeding de tailed description ofpreferred embodiments of the invention and the accompanying drawingsthereof.

Figure 1 is a schematic diagram of a regenerative gas turbine engine ofknown type and a fuel system therefor according to the invention.

Figure 2 is a partial schematic diagram illustrating a modification ofthe system of Figure 1.

Figure 3 is a longitudinal sectional view of a thermal compensatingvalve. I

Figure 4 is a transverse sectional view of the same taken on the planeindicated by the line 44 in Figure-3.

Figure 5 is an end view of the same.

Figure 6 is an end view of a cooled fuel atomizing nozzle.

Figure 7 is a side view of the same.

Figure 8 is a fragmentary front elevation view, with parts cut away, ofthe diffuser portion of the centrifugal compressor of the engineillustrating the atomizing air pickup.

Figure 9 is a sectional view of the same taken on the plane indicated bythe line 99 in Figure 8.

Referring first to Figure 1, the engine to which the invention isapplied may be described briefly. Such. an engine is described in US.patent application Serial No. 559,475, filed January 16, 1956, of commonownership with this application. The engine E comprises a centrifugalcompressor C including a rotor 10 discharging into a radial diffuser 11.Compressed air is discharged from the outer part of the diffuser into anengine case 13 within which is mounted a regenerator R including tworotary matrices or drums 14 having an axis of rotation A. Air flowsthrough the regenerator into combustion apparatus 16 which dischargesinto a turbine T; The turbine includes a nozzle discharging into a firstor' compressor turbine wheel 17 connected by a shaft 18 to thecompressor rotor. The gas discharged from the com pressor turbine passesthrough a second nozzle and a power turbine wheel 19 mounted on a poweroutput shaft 21. The turbine nozzles are supported in a case 22 mountedin a diaphragm 23 which mounts main or diaphragm seals through which theregenerator drums pass. Gas exhausted from the turbine flows through theregenerator matrices 14 into the portion of the case 13 rearwardly ofthe diaphragm which may be termed an exhaust collector, and throughsuitable exhaust gas outlets indicated at 24.

Suitable bulkheads and bypass seals are provided to insure that the gasflowing from the compressor to the combustion apparatus and from theturbine to the exhaust passes through the regenerator matrices, whichare rotated slowly about their axis when the engine is in operation.

The combustion apparatus 16 may comprise two or more flame tubes 27, twobeing illustrated. Fuel is supplied to each flame tube by a fuel nozzle26 mounted on the engine casing and projecting into the flame tube 27.within which combustion takes place.

Fuel is supplied to the engine from a tank 28 by a positive displacementpump 29, which may be driven by the engine. As illustrated, the pump isdriven by a shaft 31 geared to an accessory drive shaft 32 geared to thecompressor turbine shaft 18. The capacity of the pump is greater thanthe fuel requirement of the engine.

Pump 29 draws fuel from the tank through a line 33 and discharges itthrough a line 34, a governor valve 36,

line 37, a fuel limiting valve 38, line 39, a shutoff valve 41, line 42,and branch lines 43 to the fuel nozzles. Excess fuel is returned fromline 34 to the fuel tank through a head regulating valve 44 and returnline 46. The head regulating valve is a common component of such fuelsystems. It responds to pressure in the line 34 upstream of valves 36and 38 and to the pressure in line 39 downstream of the valves, which iscommunicated to valve 44 through the pressure line 47. Valve 44 bypassessuflicient fuel through return line 46 to maintain a substantiallyconstant pressure drop or head through the metering valves 36 and 38.Valve 41 is a shutoff valve which is closed when the engine is out ofoperation and is opened during the starting cycle of the engine to allowfuel to flow to the nozzles. As indicated, this valve is controlled by asolenoid 49 connected by a switch 51 to suitable current source 52. Anemergency relief valve 45 is provided between the pump outlet line 34and return line 46.

Governor valve 36 may be any suitable throttling valve provided with avariable speed governor actuating means. It acts to maintain the speedof turbine 17 at the desired value. As illustrated schematically, valve36 is controlled by flyweights 53 mounted on a shaft 54 coupled by gears55 to the accessory drive shaft 32. The force of fiyweights 53 isopposed by a speeder spring 56 loaded by an arm 57 coupled to a suitablepedal or manual control 58. The control may adjust the setting of thegovernor spring from a minimum setting corresponding to idling operationof the engine to a maximum setting corresponding to the maximum desiredspeed of turbine 17.

The fuel limiting valve 38 is a fuel metering device provided toregulate the fuel supply of the turbine to retain it in proper relationto the air flow through the engine so that excessive temperature of themotive fluid entering the turbine is prevented. While many arrangementsfor this purpose are known, for the purpose of the particular controlillustrated herein valve 38 is a simple throttling valve actuated by abellows or other expansible chamber motor 59 opposed by a spring 60 andconnected by a pressure line 61 to a probe 62 which picks up compressordischarge pressure from within the engine case 13. The valve is soconstructed or contoured that, as the pressure increases and the bellowsis expanded, the throttling valve opens to maintain a substantiallyconstant ratio of fuel to air. It will be apparent that the detailedstructure of the valve 38 and bellows or other motor 59 is immaterial tothis invention.

Since the governor valve 36 and limiting valve 38 are connected inseries, either may limit or regulate fuel flow to the engine.Ordinarily, in steady state operation or when the engine is decelerated,the governor valve controls. When the speed setting of the governor isincreased, the governor valve tends to open wide and the limiting valve38 acts to prevent oversupply of fuel to the engine which would resultin excessive temperature. Valves 36 and 38 may be regarded as the-normalfuel metering or fuel regulating means of the fuel control.

The limiting valve 38 is calibrated to supply a suitable quantity offuel when the engine and particularly regenerator R are hot. In thiscase, the air flowing from the compressor to the combustion apparatus isheated several hundred degrees by the regenerator and the maximum amountof fuel is that required to raise the temperature of the already hot airto the desired turbine inlet temperature.

When the engine is first started after being out of service for somewhile, the regenerator is cold, and it takes some time to heat upbecause of its high heat storage capacity. During this time, additionalfuel is needed to bring the air up to the desired temperature. This isprovided by a thermal compensating or start compensating valve 64connected in parallel with valve 38 by lines 66 and 67. The compensatingvalve is a throttling valve of small capacity controlled by atemperature responsive element 68, preferably mounted in the exhaustsection of the engine near the regenerator so as to be swept by turbineexhaust gas which has passed through the regenerator. Temperatureresponsive element 68 is also affected by heat radiated from the matrix14. When the engine is hot, element 68 closes valve 64. However, if theengine is out of service for a time sufficient to cool, valve 64 willopen when the engine is started. Under these conditions, valve 64 allowsadditional fuel to flow to the nozzles 26. As the engine heats up, thevalve 64 closes. If the engine is started after a momentary shutdown,valve 68 will respond to the high temperature of the matrix and of theexhaust gases passing through it and will remain closed.

The air supply arrangement for the fuel nozzles is also shownschematically. The fuel nozzles 26 have air supply lines 69 which arenormally supplied through a check valve 71 from a total pressure probe72 in the diffuser 11 of the compressor. The structure and installationof this probe will be described more fully; but for the present, it issufficient to point out that this probe is installed so as to take offthe maximum total pressure available in the compressor and to provide arelatively small quantity of air for fuel atomization. As a practicalmatter, two or even more probes 72 may be installed and connected inparallel to the check valve 71, depending upon the dimensions of thediffuser and the amount of air required. By recovering the velocity headof the air very effectively in probe 72, pressure may be attained whichis sufiiciently higher than the pressure within the fiame tube 27 toprovide a high velocity air stream which will serve to atomize the fuelin the nozzles 26. This pressure head results from the higher efiiciencyof pres sure recovery in the probe to that in the engine case in themain outlet of the diffuser, and also to some extent from the pressuredrops caused by flow through the matrix 14 and the wall of the flametube 27. This atomizing head is not available when the engine is beingstarted, so for starting purposes an auxiliary air compressor 73 drivenby a motor 74 and connected through check valve 76 to lines 69 isprovided. Motor 74 may be energized from a suitable power source 77 by aswitch 78. Motor 74 may, if desired, be the starting motor provided tostart the engine. When the engine has been brought up to idling speed,motor 74 may be deenergized and the atomizing air is derived from theprobe 72. The check valves 71 and 76 simply serve to prevent diversionof air supplied by either source.

The cooling arrangement for the fuel nozzles 26 involves structure whichwill be described subsequently. So far as the system shown in Figure lis concerned, each nozzle has fixed to it a cooler 79, through whichcooling fluid is circulated. While any fluid might be-used, it is bothconvenient and desirable to use excess fuel dis charged by pump 29.Nozzle cooling fuel is taken off by branch pipe 82 from the pumpdischarge line 34 and supplied through a throttle or regulating valve82, line 83, and branch lines 84 to the coolers. Fluid is dischargedfrom the coolers through lines 86 and 87 to the fuel tank. Valve 82 maybe provided to control the amount of fuel circulated for cooling, or itmay be omitted, the flow, in that case, being determined by the pressureof the pump discharge and the resistance to flow of the supply andreturn lines to the coolers.

Figure 2 is a fragmentary view of a modification of the system of Figure1 which differs from that of Figure 1 only in that the thermalcompensating valve 64 is connected in parallel with the governor valve36 as well as the limiting valve 38. This is an advantageousmodification, particularly if governor 53 has a considerable droop toits characteristic. assumed that the idle setting of the governor issuch as to cause turbine 17 to idle at 12,000 r.p.m. when the engine ishot. When the engine has just been started and the regenerator is cold,more fuel is required for idling, but for the governor to supply morefuel the engine must To illustrate this point, let it be slow down inaccordance with the characteristiccurveof the governor. Initially, theengine might run at, say, 10,000 r.p.m., gradually increasing speed asthe regenerator is heated. With the compensating valve connected inparallel with the governor, it can be calibrated to supply additionalfuel in accordance with regenerator temperature so as to compensatereasonably closely for the governor droop characteristic and thuseliminate the reduction in idle speed due to the cold regenerator.

Where this slow idling of a cold engine is not objectionable, or if thegovernor should be of substantially isochronous type, the compensatingbypass for the governor is not required. In this case, the fluid circuitshown in Figure 1 may be used.

The thermal compensating valve 64 would be connected in parallel withboth the governing and limiting valve means if both are combined so thatthe speed-responsive and air pressure responsive means control asingle'valve. Such dual actuation of a fuel controlling valve is commonin the art. However, these valves preferably are distinct, and are soillustrated in the schematic diagram for this reason and for clarity.

Referring to Figures 3, 4, and 5, the thermal compensating valveassembly 64 comprises a body or housing 101, having a flange 102provided with three holes 103 for bolts or studs by which it is fixed tothe wall of the engine exhaust casing, which has an opening over whichthe flange 102 is bolted. The temperature sensitive device 68 whichoperates'the valve is a bimetal helix projecting into the engine case.As shown most clearly in Figure 5, the outer end of the helix isreceived in a slot in a post 107 extending from the flange, where it maybe secured by a cotter key. The inner end of the helix is received in atransverse slot 108 in the end portion 109 of a valve stem 110. Valvestem 110 is rotatably mounted in a sleeve 111, pressed or otherwisefixed in a bore in the housing 101. The fuel inlet conduit 66 is coupledby conventional fittings 112 to a head or cover 113 fixed to the body101 by cap screws 114. The valve stem 110 has an enlarged head 116 whichengages the inlet end of sleeve 111. As shown best in Figure 4, onequarter of the stem, including the head, is cut away to provide alongitudinal passage 117 which extends to a point beyond a sawcut 118,out through approximately 90 of the circumference of sleeve 111. Sawcut118 communicates with a recess 119 in the housing which connects throughan outlet 121 and fittings 122 with the outlet line 67 from the valve.As will be apparent, as soon as the valve stem rotates slightlyclockwise from the position illustrated in Figure 4, the area of thepassage from the stem through the sawcut in the sleeve begins todiminish. Upon approximately 90 rotation of the stern, this passage isclosed. The temperature sensitive bimetal element 68 is configured toachieve this amount of rotation when the regenerator reaches its normaltemperature in operation of the engine. O-ring seals are mounted aroundthe stem 110, between the sleeve 111 and the body 101, and between cover113 and the body. It will be apparent that the construction illustratedis particularly suitable for a metering or throttling valve adapted topass a small amount of fluid and to be actuated accurately by atemperature sensitive element such as the bimetal coil 68.

Referring to Figures 6 and 7, these illustrate a preferred structure offuel nozzle cooling means 79 associated with a fuel nozzle 25 of anair-atomizing spray type. The nozzle 26 comprises a body 131 having aflange-132 with bolt holes through the flange for mounting the nozzle onthe engine case. Fuel is admitted to the nozzle through an inletconnection 134 which connects to the fuel line 43, and compressed air isapplied through an inlet fitting 136 which connects to the air line 69.The nozzle comprises a spray head 137 extending from the body from theend of which the fuel is sprayed into the combustion liner 27. Thecombustion liner has an opening in the wall thereof which pilots overthe-spray head 137 and against a shoulder 138. The internal structure ofthe nozzle by which the spray is .produced is not illustrated ordescribed, since it is entirely immaterial to the present invention.Such nozzles are well known to the art and may be procured. The part ofthe nozzle assembly with which the present invention is concerned liesin the cooler and its combination with the nozzle. The particular formof cooler 79 which is adapted to cooper ate with the particularconfiguration of nozzle illustrated comprises a shallow metal cup 139,the rim of which is brazed at 141 to the outer face of the nozzle body.The cooler also comprises a fluid inlet tube 84 and a fluid outlet tube86, which are small diameter metal tubes-ex tending through openings inthe lower portion of the rim of the cup and which are brazed around theentrance and brazed to the bottom surface of the cup to prevent leaksage and to secure a strong joint between the tubes and the cup. It willbe noted that tube 84 projects somewhat tangentially into the chamber142 within the cooler so as to set up a swirling flow of liquid in thecooler and that the outlet 86 extends near the top of the chamber 142 soas to trap a smallamount of fluid in the cooler:

As previously stated, tube 84 is connected to a suitable source ofliquid at low temperature which by contact with the outer wall of thenozzle body 131 greatly reduces the temperature of the nozzle, which isheated both by the hot air supplied to the combustion chamber and byexposure of the nozzle tip to radiation from the flame within thecombustion chamber. Preferably the cooling fluid is excess fueldisplaced by pump 29 and the cooling fluid is returned to the fuel tank28.

It is appreciated that fuel nozzles of a return-flow type are cooled tosome extent by the excess fuel supplied to them. However, such fuelnozzles have their disadvan tages, among them being the need for a morecomplex fuel system to balance the discharge from the nozzles. Thecooling system of this invention provides adequate cooling by verysimple structure without incurring the disadvantage of return-flownozzle systems.

Referring to Figures 8 and 9, these figures illustrate a part of theoutlet portion or diffuser 11 of an axial flow compressor of the typedisclosed in the previously mentioned application Serial No. 559,475suflicient to explain the installation of the air pick-up tube. Thecompressor case may comprise a front wall 151 and a rear wall 152, theouter portions of which are substantially flat plates between which theair flows radially and With a circumferential component of motion afterit is discharged from the compressor rotor 10. Plate 152 has arearwardly directed flange 153 and plate 151 has a flange 154 whichbolts to a portion 155 of the engine case, as by studs 156 and nuts 166distributed around the plate. A diifuser plate 157 mounted betweenplates 151 and 152 mounts vanes (not shown) which define diffusing pathsthrough the space between plate 157 and plate 152. An annular air outlet159 is defined between the flange 153 and case 155. The pick-up tube 72is of very simple structure, consisting of a length of tubing 161 whichextends through a drilled opening 162 in the flange 154. A bracket 164welded to the tube 161 is drilled to fit over one of the studs 156 andbe retained by the nutl66. As will be apparent, the open end 167 of thetube extends a short distance into the diffuser and is directedsubstantially into the direction of air flow radially andcircumferentially of the diffuser so that it receives the air with aminimum of loss of velocity head. The air entering the tube 161 isslowed to a very low velocity and the velocity head is thus recovered.Depending upon the air flow required and the dimensions of the diffuserair passage, one or more pick-up tubes 161 may be provided. The outerend of tube 161 may be provided with a suitable fitting (not shown) bywhich it is connected directly or indirectly to the check valve 71, asillustrated in Figure 1.

By thus providing an air pick-up tube so located as to convertsubstantially all of the velocity head of the air into pressure head, apressure sufficiently above that within the flame tube 27 may beprovided to successfully atomize the fuel. This makes it possible toeliminate additional compressors which have been used previously forthis purpose, except, of course, that an auxiliary compressor is neededfor starting. This auxiliary compressor 73 may be shut off as soon asthe engine is in idling op eration.

It will be seen that the fuel system of the invention is particularlywell adapted to the requirements of regenerative gas turbines such asmay be used for automobiles arid similar installations. The inventionovercomes several difliculties which have been encountered with suchfuel systems because of the heat retaining characteristics of thematrix, the high temperature of the air delivered to the combustionapparatus, and the very small fuel requirement of such an engine underidling conditions when the matrix has become hot. Certain subject matterdisclosed herein is the subject matter of our divisional applicationsS.N. 57,505 and SN. 57,506, filed September 21, 1960.

The detailed description of preferred embodiments of the invention forthe purpose of explaining the principles thereof should not be regardedas limiting the invention. Many modifications of structure may be madeby the exercise of skill in the art within the scope of the invention.

We claim:

1. A fuel system for a regenerative gas turbine engine comprising acompressor, combustion apparatus, a regenerator having high heat storagecapacity, a turbine, and an exhaust duct, the motive fluid circulatingfrom the compressor through the regenerator and combustion apparatus tothe turbine and from the turbine through the regenerator to the exhaustduct, the turbine being connected to drive the compressor; the fuelsystem comprising, in combination, a fuel pump, fuel metering valvemeans responsive to conditions indicative of normal engine fuelrequirements connecting the fuel pump to the combustion apparatus, meansconnected to the engine sensitive to a condition indicative ofregenerator temperature, and a thermal compensating throttling valveoperated by the sensitive means connecting the fuel pump to thecombustion apparatus.

2. A fuel system for a regenerative gas turbine engine comprising acompressor, combustion apparatus, a regenerator having high heat storagecapacity, a turbine, and an exhaust duct, the motive fluid circulatingfrom the compressor through the regenerator and combustion apparatus tothe turbine and from the turbine through the regenerator to the exhaustduct, the turbine being connected to drive the compressor; the fuelsystem comprising, in combination, means for supplying fuel underpressure, normal fuel control means responsive to a condition indicativeof fuel required to maintain a desired turbine temperature when theregenerator is not, the normal fuel control means connecting thesupplying means to the combustion apparatus for supplying fuel thereto,and thermal compensating fuel control means connecting the supplyingmeans to the combustion apparatus to admit additional fuel to thecombustion ap paratus when the regenerator is cold, the thermalcompensating control means including means responsive to a conditionindicative of regenerator temperature.

3. A fuel system for a regenerative gas turbine engine comprising acompressor, combustion apparatus, a re generator having'high heatstorage capacity, a turbine, and an exhaust duct, the motive fluidcirculating from the compressor through the regenerator and combustionapparatus to the turbine and from the turbine through the regenerator tothe exhaust duct, the turbine being connected to drive the compressor;the fuel system comprising, in combination, means for supplying fuelunder pressure, normal fuel control means including a fuel governorvalve means responsive to turbine speed and fuel limiting valve meansresponsive to a condition indicative of fuel required to maintain adesired turbine temperature when the regenerator is hot, the normal fuelcontrol means connecting the supplying means to the combustion apparatusfor supplying fuel thereto, and thermal compensating fuel control meansconnecting the supplying means to the combustion apparatus to admitadditional fuel to the combustion apparatus when the regenerator is"cold, the thermal compensating control means including a throttlingvalve and temperature-sensitive means responsive to a conditionindicative of regenerator temperature controlling the valve.

4. A fuel system as recited in claim 3 in which the throttling valve isconnected in series with the governor valve means.

5. A fuel system as recited in claim 3 in which the throttling valve isconnected in parallel with the governor valve means.

6. A fuel system for a regenerative gas turbine engine comprising acompressor, combustion apparatus, a regenerator having high heat storagecapacity, a turbine,

and an exhaust duct, the motive fluid circulating from the compressorthrough the regenerator and combustion apparatus to the turbine and fromthe turbine through the regenerator to the exhaust duct, the turbinebeing connected to drive the compressor; the fuel system comprising, incombination, means for supplying fuel under pressure, normal fuelcontrol means responsive to a condition indicative of fuel required tomaintain a desired turbine temperature when the regenerator is hot, thenormal fuel control means connecting the supplying means to thecombustion apparatus for supplying fuel thereto, and thermalcompensating fuel control means connecting the supplying means to thecombustion apparatus in parallel with the normal fuel control means toadmit additional fuel to the combustion apparatus when the regeneratoris. cold, the thermal compensating control means including a throttlingvalve and temperature-sensitive means responsive to a conditionindicative of regenerator temperature controlling the valve, thetemperature-sensitive means being located to respond to heat radiationfrom the regenerator and to temperature of gas exhausting from theturbine downstream of the regenerator.

References Cited in the file of this patent UNITED STATES PATENTS2,489,683 Stalker Nov. 29, 1949 2,541,108 Sforzini Feb. 13, 19512,593,849 Clarke et al. Apr. 22, 1952 2,695,498 Szydlowski Nov. 30, 19542,741,086 Machlanski Apr. 10, 1956 2,780,061 Clarke Feb. 5, 19572,862,355 Davis et al. Dec. 2, 1958 an; b m-H UNITED STATES PATENTOFFICE CERTIFICATION OF CORRECTION Patent more $976,683 March 28 r 1961EugeneEmFlanigan et al.

It is hereby certified'that error Appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 4, line 18, after "but" insert a comma; line 54, for "82" read.81 column 7, line 60 for not read hot Signed and sealed this 29th day ofAugust 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

